This invention relates to an image sensor, more particularly to a contact-type image sensor for actual size reading, i.e., one-to-one scanning of an information medium such as a document.
A contact-type image sensor providing actual size reading of subject copy has the advantage of permitting an image scanning apparatus such as a facsimile apparatus to be made smaller, because no reducing optical system is needed.
An example of the prior art in this field is given in the article "Koro ga mijikai mitchakugata imeji sensa OA kiki kogataka no kirifuda ni" (Contact-type image sensors with short light paths--the key to smaller OA equipment) in Nikkei Mechanical, Dec. 1, 1986, pp. 71-78.
The prior art configuration will be described with reference to the drawings.
FIG. 1 is a cross-sectional view of a prior art contact-type image sensor. As shown in this drawing, light from an LED array 1 illuminates a document 2, and the reflected light passes through a rod lens array 3, forming an image on the photosensor area of a contact-type image sensor 4.
A contrasting device is the direct-contact-type image sensor in FIG. 2, in which light from an LED array 6 acting as a light source passes through a glass substrate 9, illumination windows 10, and a transparent dielectric protective film 7 several microns in thickness, illuminates a document 5, then is reflected back through the transparent dielectric protective film 7 and sensed by a photoelectric transducer made of amorphous silicon 8. The numeral 11 is a metal film acting as an electrode, 12 is a transparent electrode, 13 is a common electrode, and 14 is a roller.
Another example of publication showing similar contact-type image sensors is the paper "Kanzen mitchakugata amorufasu shirikon imeji sensa" (A direct-contact-type amorphous silicon image sensor) in the proceedings of a symposium on advances in amorphous silicon devices held by The Society of Electrophotography of Japan, (May 24, 1985, pp. 53-56).
Since the contact-type image sensor illustrated in FIG. 1 has a rod lens array 3, it is about 20 mm in height as shown in FIG. 1, cannot be reduced in size, and requires an assembly manufacturing process.
In the direct-contact-type image sensor shown in FIG. 2, to improve the resolution the roller 14 must feed the document 5 in pressure contact with the transparent dielectric protective film 7. A resulting problem is that friction arises between the paper document 5 and the transparent dielectric protective film 7, the transparent dielectric protective film 7 becomes charged, and its electric field causes malfunction of the photosensor elements. The cause of the charge induced by friction between the transparent dielectric protective film 7 and the paper is the high resistivity (&gt;10.sup.15 ohm.multidot.cm) of the film of, for example, SiN.sub.x used as the transparent dielectric protective film.